1. Field of Invention
This invention relates to rolled H-shape steel products (H-shapes) having a small qualitative variation within each product and also having small qualitative variations between products, with each steel product having high strength and high toughness. This invention also relates to a method of manufacturing H-shapes.
2. Description of Related Art
H-shapes are used in various industrial fields, such as construction, marine structures, shipbuilding, storage tanks, civil engineering and construction machinery. For a long time, people have tried their best to improve the characteristics of H-shapes so as to obtain a higher strength and a higher toughness. Particularly, in recent years, there has been a demand for an H-shape manufactured such that its various characteristics are uniform along its thickness direction, and that the H-shape characteristics are the same from one product to another.
Further, with the development of construction techniques for high buildings, it has been reported that many construction designs are capable of absorbing vibration energy caused by deformation of a building during a large earthquake, thereby preventing the building from collapsing (see, e.g., Iron and Steel, 1988, No. 6, pp. 11-21). According to these construction designs, the skeleton (framed structure) of a building is allowed to collapse in a predetermined manner during an earthquake, thereby preventing the building itself from collapsing due to the plasticity of the materials forming the skeleton.
With the above construction designs, it is assumed that the skeleton of a building should behave in a predetermined manner, as designed by its designer, during an earthquake. Namely, it is necessary for the building designer to know the yield strength of steel materials forming each column and each beam of the building. For this reason, it is absolutely important that a steel product, such as H-shape for use in forming each column and each beam, have characteristics which are uniform within each column and beam, and which are also uniform from one product to another. In other words, severe problems will occur if an H-shape has characteristics which are not uniform within the product itself, and if the characteristics of one product are different from those of another.
However, with regard to some steel products used in civil engineering, construction and shipbuilding, these steel products should have a high strength and a high toughness. Accordingly, these steel products are usually produced by a controlled rolling and controlled cooling method, known as the Thermo Mechanical Controlled Process (TMCP method).
However, when a steel product having a thickness of about 40 mm is manufactured using the TMCP method, during the cooling process conducted after the rolling treatment, the cooling rate will be different from place to place in the thickness direction of a given steel product, and will also be different from one steel product to another. As a result, the structure of a finally obtained steel product will not be uniform everywhere within the product, and the microstructure of one steel product will be different from that of another. The material quality of each given steel product will be different from place to place in the thickness direction of the product, and the material quality of one steel product will be different from that of another.
Further, when the hardenability of each steel product is to be increased, the welding crack sensibility index, which is an index of weldability (hereinafter simply referred to as Pcm), will be undesirably increased. Namely, there has been a problem that the toughness of each welding heat-affected zone (hereinafter simply referred to as xe2x80x9cHAZxe2x80x9d) will deteriorate.
In the past, steels having a high tensile strength over 570 MPa were produced mainly through a process including reheating, quenching and tempering, to thereby obtain a finely tempered martensite structure. However, the process including reheating, quenching and tempering is too expensive.
In order to solve the above problem, there have been suggested improved steel products that have a small qualitative variation within each product, and also among a plurality of steel products, and that are capable of inhibiting the deterioration of the toughness of the HAZ.
Also, there have been suggested several improved processes for manufacturing the steel products. These steel products and the manufacturing processes are disclosed in Japanese Unexamined Patent Application Publication Nos. 8-144019, 9-310117, 10-72620. The techniques disclosed in these publications all form steel structures that contain bainite as their main microstructure, irrespective of variations in their cooling rates.
Actually, the techniques disclosed in the above publications are all based on a newly discovered fact indicating that the occurrence of qualitative variation of steel products is caused by microstructural changes because of the difference in cooling rate from place to place within a piece of steel as it is being cooled. Therefore, the above-mentioned techniques have tried to solve the above-mentioned problems by preparing an improved steel composition that is useful in preventing the microstructure from changing, without having to consider any change in the cooling rate. It has been reported that the above-mentioned techniques are so established that an appropriate amount of elemental B is added into an extremely low carbon steel, or into a high Mn steel, to make it possible to obtain a microstructure containing bainite as its main component, and with its composition not depending upon the cooling rate in the cooling process, thereby obtaining steel products having little qualitative variation within each steel product and also among different steel products. Further, the above-mentioned techniques also attempt to reduce C content, so as to reduce Pcm, thereby improving the weldability of each steel product.
However, the techniques disclosed in Japanese Unexamined Patent Application Publication Nos. 8-144019, 9-310117 and 10-72620 mainly relate to H-shapes having a flange thickness of more than 50 mm, and to thick steel plates having a thickness of 50 mm or more, assuming that a heating treatment after rolling is necessary. Indeed, the above mentioned techniques are suitable for use in manufacturing H-shapes having a thinner flange thickness. However, when it is desired to improve the productivity and economics in steel manufacturing, these techniques still need to be further improved, so as to improve the composition of each steel product, and to improve some relevant manufacturing methods, thereby making it possible for each steel product to obtain a high strength and a high toughness. By virtue of such further improvement, it is possible for H-shapes having the above-described thin size to obtain a fine steel structure by rolling treatment, i.e., beneficial from a rolling refinement of the structure.
In recent years, as an earthquake proof material, H-shapes having the above-described thin size have become increasingly used. Namely, up to the present time, it has been required that H-shapes having the above-described thin size should have a further higher strength and a further higher toughness, and be manufactured at a lower cost.
It is an object of this invention to provide an improved rolled H-shape which has a large tensile strength, and a high strength and a high toughness.
It is a separate object of this invention to provide an improved method for manufacturing the improved rolled H-shape having a high productivity and a high strength, which can be produced using alloy components that are cheaper than conventional ones, so that the steel product can be manufactured at a lower cost.
Namely, embodiments of this invention provide high productive and high strength rolled H-shapes having a tensile strength of 500 to 700 MPa, and comprising: 0.014 to 0.05 wt. % of C, 0.1 to 1.0 wt. % of Si, 1.0 to 1.8 wt. % of Mn, 0.030 wt. % or less of P, 0.020 wt. % or less of S, 0.1 wt. % or less of Al, 0.0003 to 0.0040 wt. % of B, 0.006 wt. % or less of N, 0.03 to 0.1 wt. % of Nb, 0.005 to 0.04 wt. % of Ti, and the balance Fe and unavoidable impurities. Further, the high productive and high strength rolled H-shape of embodiments of this invention can further comprise from 0.0005 to 0.0100 wt. % of Ca and have flange portions with a thickness of 40 mm or less.
Further, this invention also provides a method for manufacturing the high productive and high strength rolled H-shape having, in embodiments, a tensile strength of 500 to 700 MPa. The method comprises subjecting raw steel materials to reheat treatment, and then to a break down rolling, a rough universal rolling and a finishing universal rolling, thereby obtaining the H-shape.
The raw steel materials can contain the above listed components, with the balance of Fe and unavoidable impurities. In embodiments, the reheating temperature is from 1150 to 1320xc2x0 C. In embodiments, in the rough universal rolling, the accumulated reduction at a rolling temperature of 950xc2x0 C. or lower is at least 5%, with each working strip being reversed fast. In embodiments, in the finishing universal rolling, the rolling temperature is 750xc2x0 C. or higher. In embodiments of the method of this invention, in the rough universal rolling, the total stopping time period at the reverse operation is set to be 120 seconds or less, and the accumulated reduction at a rolling temperature of 950xc2x0 C. or lower is 50% or less. Alternatively, in embodiments, the products are air cooled between the rough universal rolling and the finishing universal rolling, and after the finishing universal rolling.